The Bonnet Carre Spillway is a flood control operation in the Lower Mississippi Valley. Located in St. Charles Parish, Louisiana - about 12 miles (19 km) west of New Orleans - it allows floodwaters from the Mississippi River to flow into Lake Pontchartrain and thence into the Gulf of Mexico. The construction of the spillway was completed in 1931.
The Bonnet Carre Spillway consists of two basic components: a control structure along the east bank of the Mississippi River and a floodway that transfers the diverted flood waters to the lake. The control structure is a mechanically controlled concrete weir which extends for over a mile and a half parallel to the river. When opened, the control structure slightly restricts the flow of the river (at the structure’s location) toward its main channel, thereby causing it to rise in elevation just high enough to flow into the diversion channel; and, with sufficient elevation (or head), to carry the overflow volume into Lake Pontchartrain. The lake’s opening to the gulf is sufficient to absorb and dissipate any conceivable volume of flood flow. Thus, the flood surcharge portion of the water from the Mississippi is divided between the main river and the diversion channel; with the surcharge bypassing the New Orleans metropolitan area, resulting in the Mississippi being lower (through that area) than it could have been; and reducing the stress on the area's levees that line the river. Confined by guide levees, the floodway stretches nearly six miles (10 km) to Lake Pontchartrain, with a design capacity of 250,000 cu ft/s (7,100 m3/s). The spillway is crossed by U.S. 61 and Interstate 10.